Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
During some conditions, excessive vacuum can build inside the evaporative emission control system. For example, due to degradation of a canister purge solenoid or a canister vent solenoid, or due to a restriction in the system's fresh air line, fuel tank vacuum levels may become excessive, potentially harming the fuel tank. While evaporative emission control systems may include hardware, such as fuel caps, to relieve excess vacuum, there may be conditions where relief is not provided due to hardware malfunction. Consequently, fuel tank vacuum level may rise to dangerous levels. In addition to costly fuel tank repairs and an increase in warranty, this can also result in increased operator dissatisfaction.
Fuel tank damage can be reduced if the trapped vacuum can be vented as soon as an elevated vacuum level is observed. Toward this end, US Patent Application No. 2014/0230795 A1 teaches opening a canister purge valve to dissipate fuel tank vacuum to an engine intake manifold while the engine is not combusting, or alternatively, opening a canister purge valve to dissipate fuel tank vacuum to an engine intake manifold while the engine is combusting and the likelihood of air/fuel ratio error is low. However, the inventors herein have recognized an issue with the above approach. In the case wherein the engine is combusting and the likelihood of air/fuel ratio error is high, fuel tank vacuum may not be relieved on time.
The inventors herein have recognized that fuel tank vacuum may be relieved during engine operating conditions wherein the risk of air/fuel ratio error is high by treating fuel tank vacuum relief as a disturbance to air/fuel ratio control. In one example, this may be achieved by a method for a fuel system coupled to an engine, comprising: in response to a fuel tank vacuum level above a threshold or a rise in fuel tank vacuum level being higher than the threshold rate, opening a canister purge valve to dissipate the fuel tank vacuum level into an engine intake manifold while the engine is combusting, and adjusting an amount of intake air inducted into the intake manifold based on the fuel tank vacuum level, such that manifold pressure is minimally affected while fuel tank vacuum is relieved. In this way, a feedforward compensation approach may be used such that fuel tank vacuum levels may be reliably returned to safer levels even under conditions wherein the risk of air/fuel ratio error may be high.
In one example, it may be determined that fuel tank vacuum needs to be vented to reduce potential fuel tank damage in a hybrid vehicle. Therein, a canister purge valve (CPV) may be opened to route intake air from downstream of an intake throttle to a fuel tank while an engine is not combusting, for example during a condition wherein the hybrid vehicle is being propelled solely by battery power. During other conditions, such as when the hybrid vehicle is propelled by the engine, the canister purge valve may be opened to route intake air from downstream of the intake throttle to the fuel tank while the engine is combusting, and an amount of intake air inducted into the intake manifold may be accordingly adjusted based on a determined amount of the intake air that is routed from downstream of the intake throttle to the fuel tank.
In this way, by monitoring changes in vacuum level of an isolated fuel tank, fuel tank vacuum build-up can be detected and addressed before fuel tank degradation is incurred. By venting the excess vacuum to the engine while the engine is not combusting, air-fuel errors are averted. Alternatively, by dissipating the excess vacuum to the intake while the engine is combusting and adjusting an amount of intake air inducted into the intake manifold based on a determined amount of the intake air that is routed from downstream of the intake throttle to the fuel tank, fuel tank vacuum may be relieved while air/fuel ratio errors may be reduced. By rapidly and reliably addressing excess fuel tank vacuum, fuel tank degradation can be reduced and fuel system integrity can be better maintained.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.